The changes of electrical polarization of primary afferent terminals in the lumbosacral spinal cord have been investigated on immobilized, decorticated and spinal cats during fictive locomotion. Fictive locomotion was spontaneous or provoked by stimulation of either dorsal root or dorsal funiculi in the lumbar segments. The activation of the locomotor generator and appearance of fictive locomotion were always associated with a sustained dorsal root hyperpolarization. On the background of this positivity periodic negative dorsal root potential oscillations appeared synchronously with efferent discharges in motor hind-limb nerves. These periodic waves of primary afferent depolarization occurred in phase in different ipsilateral lumbosacral segments. On the contralateral side the periodic changes in dorsal root potential were out of phase during fictive stepping and in phase during fictive galloping. The use of Wall's technique has shown that tonic and periodic changes in dorsal root potential reflect the changes occurring in polarization of central terminals of cutaneous and muscle (Ia and Ib) groups of afferent fibres. It is concluded that the level of electrical polarization of primary afferent terminals is determined directly by the activity of the spinal locomotor generator; activation of the generator is followed by hyperpolarization of primary afferent terminals. By so modulating the polarization of afferent terminals, the locomotor generator can perform tonic and phase-dependent selection of afferent information.